Field of the Invention
The present invention relates to an R-T-B rare earth sintered magnet and a method of manufacturing the R-T-B rare earth sintered magnet, and particularly, to a method of manufacturing an R-T-B rare earth sintered magnet having excellent magnetic properties.
Priority is claimed on Japanese Patent Application No. 2013-089744, filed on Apr. 22, 2013, and Japanese Patent Application No. 2013-151073, filed on Jul. 19, 2013, the contents of which are incorporated herein by reference.
Description of Related Art
Hitherto, R-T-B rare earth sintered magnets (hereinafter, may be referred to as “R-T-B magnet”) have been used in voice coil motors of hard disk drives and motors for engines of hybrid automobiles and electric automobiles.
In general, in R-T-B magnets, R is Nd, a part of which is replaced by other rare earth elements such as Pr, Dy, and Tb. T is Fe, a part of which is replaced by other transition metals such as Co and Ni. B is boron and a part thereof can be replaced by C or N.
Normal R-T-B magnets have a structure constituted mainly by a main phase consisting of R2T14B and an R-rich phase which is present at the grain boundaries of the main phase and has a higher Nd concentration than the main phase. The R-rich phase is also referred to as a grain boundary phase.
Japanese Patent No. 3405806 proposes a method of infiltrating a melted alloy for infiltration into a compact of a powder of an alloy for an R-T-B magnet, as a method of improving the coercivity of the R-T-B magnet.
PCT International Publication No. WO2011/070827 proposes a manufacturing method including: pressurizing a mixed raw material made by mixing a magnet raw material and a diffusion raw material to form a compact; and heating the compact.
Japanese Unexamined Patent Application, First Publication No. H7-176414 proposes a manufacturing method including: molding a mixture of a powder of a mother alloy for a main phase and a powder of a mother alloy for a grain boundary phase; and sintering the resulting molded product.
When R-T-B magnets are used at a temperature equal to or higher than the room temperature, coercivity (Hcj) decreases with an increase in temperature. The coercivity (Hcj) of R-T-B magnets is improved when heavy rare earth elements such as Dy and Tb is contained. Therefore, in conventional R-T-B magnets, a heavy rare earth element is added to achieve coercivity in an operation temperature range. In addition, it is required to further improve the coercivity of R-T-B magnets in order to increase the efficiency of generators or motors.
However, heavy rare earth element can be mined only in the limited place. Furthermore, heavy rare earth element reserves are smaller than reserves of light rare earth elements such as Nd and Pr. Therefore, when a large amount of heavy rare earth elements is used, the balance between the demand and the supply of heavy rare earth elements is disrupted and this leads to a sharp rise in price. Moreover, it becomes difficult to stably secure a required amount. Therefore, it is required to provide R-T-B magnets having high coercivity without using heavy rare earth elements as much as possible.